1. Field of the Invention
The present invention relates to an improved physical vapor deposition (PVD) process for forming mirror layer with improved reflectivity.
2. Description of the Prior Art
Micro-display devices such as digital light processing (DLP) or LCOS projection units require a mirror layer to reflect image light signals. Such mirror layer is typically made of thin metal layers or high-reflectivity materials. Thin aluminum mirror layer is commonly used because aluminum is relatively inexpensive compared to other mirror materials, and because of its relatively higher reflectivity. To form an aluminum mirror layer of a micro-display device, pure aluminum is evenly sputtered onto the wafer surface using physical vapor deposition (PVD) methods
As known in the art, PVD is mainly a physical, rather than chemical, process. In PVD, atoms of a heavy, but inert gas, typically argon, are electrically accelerated toward a target of pure metal. These atoms chip off or sputter the target material, atom by atom. The sputtered metal having high temperatures lands on the wafer surface, where it forms a solid layer of metal, and increases the temperature of the wafer. Thereafter, this metal layer is patterned and etched to form pixel mirrors of a semiconductor image device.
The aforesaid aluminum mirror layer is deposited in PVD process chambers which is capable of providing a vacuum environment with high substrate temperature (380-500° C.), thus the metal atoms deposited on the wafer become crystallized grains. It is often desirable to form an aluminum layer with relatively higher reflectivity through spectrum. The prior art aluminum mirror layer deposited using traditional PVD process has a drawback in that the formed aluminum mirror layer has an obvious abrupt drop of reflectivity at a wavelength around 380 nm, which is difficult to improve.